1. Field of the Invention
This invention relates to methods of casting Aluminum-Silicon alloys and more particularly, to a process for nodulizing silicon crystals in the casting of Al-Si alloys.
2. Background
The eutectic Al-Si alloy as conventionally cast, is an excellent wear-resistant light metal which is widely used for moving and easily worn-out parts in automobiles, airplanes and spacecraft. However, there has long been an industry need for Al-Si alloys having better resistance to wear coupled with higher tensile strength and other improved mechanical properties. This improvement is required particularly to meet present day demands of equipment such as automobile engines and aircraft mechanical controls, as well as for many industrial fabrication applications.
In pursuit of this need, over the past several decades, metallurgists have paid much attention to AL-Si alloys, particularly to the effects achieved by varying the alloy silicon content. When an increase is made in its silicon content, the alloy resistance to wear rises. However, the silicon crystals tend to become coarse and angular, which leads to a weakening of the alloy's mechanical properties, particularly ductility, castability and machinability of the alloy. To overcome the problems associated with increased silicon content, metallurgists in several countries have investigated methods of refining the silicon crystals in the alloy in order to improve the properties and microstructure of the Al-Si alloy, and further to obtain nodular silicon.
In the 1950's and 1960's decades, many different impurities such as Na, Sr, rare earth elements (RE) and others or their combination, were added to the molten Al-Si alloy prior to casting in attempts to obtain nodular silicon crystals in eutectic or hypereutectic alloys. These efforts were not successful, resulting in a poor quality Al-Si alloy having a tendency toward porosity and being unsuitable for industry.
Other methods have been tried to nodulize silicon crystals in a eutectic Al-Si alloy (Si&lt;12%) at high temperatures near the melting point. However, this approach produced only coarseness and clusters of nodular silicon. It was found that high temperatures will not nodulize or blunt the primary silicon crystals, and the effort was dropped as being unsuccessful.
Of late, several patents to improve the quality of high-silicon (hypereutectic) Al-Si alloys have been issued. Although these patented Al-Si alloys offer generally better properties than existing conventional high-silicon alloys, their silicon crystal morphology exhibits an angular form and tends to be clustered, not favoring wear resistance, so improvement is marginal at best in a critical. characteristic such as wear.
Therefore, there remains an important industrial need for a process for nodulizing silicon in casting aluminum-silicon alloys that will produce alloys having significantly improved mechanical properties of resistance to wear, tensile strength, low porosity and machinability coupled with low cost.